Groups-dev drupal drush update to 6.0.0

Bug #1228206 requires drush 6.0.0 instead of 5.9.0 to apply
local patch files.

Change-Id: Ie25eec114d0a4ffe9c328c2d74972bb3512510a0
Merge "Groups-dev drupal drush update to 6.0.0"